1. Field of the Invention
The present invention relates generally to alternating current power distribution systems and more specifically to an automatic power factor control device for such systems.
2. Description of the Prior Art
When inductive loads are added or subtracted to a power distribution system, they can have an adverse effect on the power factor which results in excess consumption of electrical energy. Due to the relatively high costs of electrical energy, this excess consumption can substantially increase the cost of operating inductive equipment such as electrical motors or inductive heaters.
It is well known to those skilled in the art that banks of capacitors can be added or subtracted to the power distribution system in order to improve the power factor. These prior art devices typically include mechanical contactors for switching the capacitor banks into or out of the system. These mechanical contactors cause undesirable electrical interference and electrical transients.
One attempt to solve the problems associated with power factor control is described in U.S. Pat. No. 4,356,440 by Curtis et al., assigned to the Charles Stark Draper Laboratory, Inc. and entitled, "Power Factor Correction System". The '440 patent discloses a discrete-time, closed loop power factor corrector system that controls the coupling of a delta-connected switched capacitor array to a 3- or 4-wire power line which may have time-varying, unbalanced, inductive loads. For inductive loads that cannot be exactly compensated with a delta-connected capacitance, the corrector system minimizes the total RMS reactive current drawn from the power line.
Another attempt to solve the power factor problem is disclosed in U.S. Pat. No. 4,348,631, Gyugyi et al., assigned to Westinghouse Electric Corp., and entitled, "Static VAR Generator". The '631 patent discloses a device for inserting capacitance into an AC network for power factor correction or voltage regulation that minimizes transient disturbances to the system. Current surges and voltage transients that are normally associated with connection of capacitors into an AC network are minimized by predetermining an amount of inductance that must be inserted to suppress these transients and simultaneously inserting the inductance into the network with the capacitance.